The emission spectra of a representative group of inorganic solid phosphors have been measured at temperatures as low as 4°K. Of the phosphors investigated, only those having “edge” emission show a tendency to exhibit a line spectrum at low temperatures; the ordinary impurity activated materials show little or no change in emission below 100°K. It is improbable that the breadth of emission is due to the interaction of the luminescent center with impurities or lattice defects since neither cold work nor variation in concentrations of impurities affects the breadth of emission. Possible explanations of the broad band, low temperature emission are the existence of a zero-point vibrational energy of a nonphotoconducting luminescent center in its excited state; and, in the case of photoconducting phosphors, the distortion of exciton and conduction levels near the luminescent center.
CLIFFORD C. KLICK, "Low Temperature Luminescence of Inorganic Solids," J. Opt. Soc. Am. 41, 816-822 (1951)